A hybrid of the optical and magnetic information storage systems, so called magneto-optical information storage systems, appears to have the potential to not only to increase storage capacity but also to resolve the problem of erasure of optical information and rewrite new optical information. In most previous optical systems it has not been possible to erase and rewrite information. It has been estimated that the theoretical upper limit of the storage capacity of such systems can be as high as 300 megabytes per square inch of media. In practice on a 5.25 inch disc, storage capacities as high as 400 to 800 megabytes can be expected.
Often, all of the components necessary to detect data in a magneto-optical system are included in a so-called optical head. As will be appreciated, in order to access data, the optical head is moved radially by an actuator mechanism across the surface of the rotating disc. Unfortunately, one obstacle to commercial acceptance of such magneto-optical systems is the relative slowness by which information can be retrieved compared to contemporary rigid disk magnetic storage systems. The primary factor contributing to the slow access performance of present optical storage systems is the mass of the moving optical head assembly. As will be appreciated, the greater the mass of a device for reading from or writing to an optical disc, the more difficult to accelerate such a device in relation to precise locations on a rotating disc.
One scheme for reducing the moving mass of the optical system has been to split the optical assembly into a subassembly of fixed components which remain stationary relative to the actuator mechanism and a subassembly of moving components of minimum mass and maximum robustness. As used herein such a system is referred to as a split optical system.
When reading information from or writing information onto a magneto-optical disk, or any optical disk, it will be necessary to maintain the radial position of the light beam focused by the objective lens on track as the disk rotates. Such an operation is known as track following. Track following requires the generation of a radial position (tracking) error signal. It will also be appreciated from the above that because relatively small magnetic domains will be recorded, read and erased, it is important to maintain a focused spot of light on the desired track. Maintaining the focus of the light beam requires the generation of a focus (axial) error signal. Each of these signals, the tracking (radial position) error signal and the focus (axial position) error signal, can be calculated based on signals generated by segmented detectors whose outputs are differenced (subtracted) in various ways to produce these error signals.
Light reflected from a grooved magneto-optical disk directed onto the detectors will form a sheared interferogram. That is, when light is focused on a spot on grooved media, such as that used in optical and magneto-optical disks, the reflected light contains a series of orders of diffraction each having an axis deviated from the central axis. These diffraction orders normally overlap producing the sheared interferogram. When sampled properly, the sheared interferogram can be used to generate the tracking error signal. The focus error signal can be derived in a varied assortment of ways.
In one method of focus detection, the detector in effect senses the diameter of the reflected beam of light, i.e. an illuminated spot, including the sheared interferogram. Accordingly, focus can be determined by sensing the size of the spot formed by the intersection of the detector with the converging beam. By using a detector shaped like an elongated I (a so-called "I" detector) in a differential detection scheme, focus is determined in accordance with the following formula: EQU FE=(A+D)-(B+C)
where FE is the focus error signal and A, B, C and D represent distinct sections in such "I" detectors. This method is sometimes also called "one-dimensional spot size detection".
In addition to the "I" type detector, quadrant type detectors have also been proposed for use in determining tracking error signals and focus error signals. See for example, U.S. Pat. Nos. 4,773,053 - Gottfried, 4,797,868 - Ando and 4,779,250 - Kogure, et al.; and Lee, Wai-Hon, "Optical Technology For Compact Disk Pickups", Lasers and Optronics, pp. 85-87 (September 1987). The problem with such prior techniques for generating tracking error and focus error signals, particularly where split optical components are utilized, is that the system is vulnerable to errors which originate in non-uniform or changing spatial distributions of optical power in the illuminated spot. Such non-uniform or changing spatial distributions can be referred to as "pattern noise". One example of pattern noise is the sheared interferogram described above. Such pattern noise is important because any redistribution of optical power within the illuminated spot, which is not the result of de-focus and which does not maintain an exact balance of optical power between inner and outer elements of the "I" detector, will cause an incorrect indication in the state of focus in the optical system. Such pattern noise can be caused by diffraction from surfaces which define the edge of the optical beam, dust, partial obscuration of the reflected light beam, or by the interference of diffracted orders reflected from the grooved media surface. This pattern noise is intrinsic to, or at the very least difficult to remove from, the beam of light reflected from the optical surface. It can degrade the performance of an optical system to an unacceptable level.
Previously, a number of schemes have been proposed for generating a focus error signal which is uncorrupted by pattern noise. For example, it has been proposed to generate a differential focus error signal by placing spot size detectors on opposite sides of the focal point of a positive lens, as shown in FIG. 1. In such a detection scheme, two light beams are created from an original reflected light beam by means of a semi-transparent, semi-reflective beam splitter. The first detector is placed in the converging portion of the first beam, while the second detector is placed in the diverging portion of the second beam. Each detector creates a focus error signal in the manner described above. However, the slope of the error signal generated by the first detector differs in sign and magnitude from that developed by the second detector. The algebraic difference of these two error signals is a net, differential focus error signal with pattern noise rejection properties.
As shown in FIG. 1, a collimated beam of light is focused by lens 10. The focused beam of light is split by beam splitter 12 into first and second beams 14 and 16. An "I" detector 18 is placed in the convergent portion of beam 14. A second "I" detector 20 is positioned in the divergent portion of beam 16 by positioning beyond focal point 22. Focal point 22, is of course the focal point associated with lens 10. This focus sensing scheme has the virtue of rejecting pattern noise insofar as the two detector irradiance patterns map onto one another.
A problem with the detector scheme of FIG. 1 is that the error signal from the second detector 20 is strongly dependent upon path length between the objective lens and the detectors. Clearly, such a system may not be used in the above described split optical component systems.
Consequently, a need still exists for an apparatus and method capable of generating focus and tracking signals which minimize the effects of pattern noise upon the focus error signal and further which are capable of use in split optical systems.